iTunes is designed by the Junk Drawer Method: when enough cruft
has built up that somebody tells the team to redesign it, while
also adding and heavily promoting these great new features in the
UI that are really important to the company’s other interests and
are absolutely non-negotiable, the only thing they can really do
is hide all of the old complexity in new places. […]

I have plenty of plausible theories on why iTunes didn’t get the
iCloud Photos treatment — why Apple Music was bolted onto this
ancient, crufty, legacy app instead of discontinuing iTunes,
dropping its obsolete functions, and starting fresh with a new app
and a CloudKit-based service. (Engineering resources, time to
market, iPods, Windows, and people with slow internet
connections.)

Exactamundo. iCloud Photos gets right everything that Apple Music gets wrong. Like Marco, I can imagine many reasons why Apple took a different route with music than the clean-slate approach they took with photos. I’m not in a position to judge what Apple should have done. All I’m saying is that the difference in results is stark. I understand the design and purpose of Photos (the app) on both Mac and iOS, and I understand how iCloud Photo Library is supposed to work. And, for me — and seemingly, almost everyone — that’s how iCloud Photo Library does work. You sign up, you enable it on all your devices, you wait for the initial sync to finish, and boom — now all your photos are available on all your devices, all the time. I don’t think this would have worked out as well if they had kept going with iPhoto on the Mac. They needed the clean break — both in terms of design and in terms of engineering.